1. Field of the Invention
The present invention relates to the field of telecommunications, particularly radiocommunications, and even more particularly to mobile radiocommunications networks, viz. cellular radiocommunication networks, like mobile telephony networks of the second generation (GSM—General System for Mobile communications—networks) and of the third generation (UMTS—Universal Mobile Telecommunications System—networks). Specifically, the invention concerns a method for building neighboring cells lists to be used by mobile communications terminals to accomplish hand-over from one cell to another.
2. Description of the Related Art
Cellular networks are formed of a plurality of network cells which, altogether, provide radio coverage of a geographic area of interest. It is possible to ideally subdivide said geographic area into a discrete number of elementary area elements or “pixels” (e.g., square area portions of some tens meters of side); each cell is defined as the subset of pixels that are “served” by the radioelectric signal irradiated by a given transmission station (in the GSM, a BTS—Base Transceiver Station—or a “Node B” in the UMTS).
A characterizing feature of cellular networks is that they allow user mobility. The passage of a generic user, having a mobile communications terminal registered to a cellular network, from one cell of the network to another is handled by means of a procedure referred to as “hand-over”. If the two cells belong to a same system (i.e., they are both cells of a GSM or UMTS network), an “intra-system hand-over” is said to take place; on the contrary, in case of heterogeneous networks (e.g., implementing both the GSM and the UMTS systems), the passage of the user from the cell of one system (e.g., a GSM cell, or a UMTS cell) to a cell of another system (a UMTS cell or, respectively, a GSM cell) is called “inter-system hand-over”.
The hand-over procedure is invoked when the level and/or quality of the radio communication channel currently supporting the communications of a user within a cell worsen below a predetermined minimum level, while the level of the signal reaching the user terminal and coming from a neighboring cell is sufficient to sustain the communication. In such a case, the network causes the communications with that user to migrate from the radio channel used so far and belonging to the cell of origin to a new radio channel, belonging to the neighboring, destination cell.
In order to successfully accomplish the hand-over, the destination cell needs to belong to a list of cells which the user mobile terminal exploits, while connected to the cell of origin and engaged in a call, to conduct a signal measurement campaign. When the mobile terminal connects to a network cell, the network provides to that terminal the list of neighboring cells (the so-called “NeighBoring Relations or NBR set) that the network knows being neighbors to the cell the terminal is connected to. The mobile terminal, while engaged in a call and communicating with the cell it is currently connected to, measures the level of the signals it receives from the other cells in the NBR set, and reports the measurement results to the network, so that the latter is kept constantly informed of when, and towards which destination cell, the hand-over is to be performed. Differently, when in “idle mode”, the communication is mainly unidirectional, from the network to the terminal.
The NBR set may include cells of the same system (GSM, UMTS), geometrically adjacent to (i.e., cells having boundaries in common with) the cell the mobile terminal is currently connected to (“intra-system adjacency relationships”), as well as cells of other systems that are spatially close to the cell the mobile terminal is currently connected to (“inter-system adjacency relationships”).
The NBR sets for the various network cells are determined in the network planning phase, and are then stored in the network apparatuses, ready to be provided to the terminals of users that connect to the network.
The network planning is usually made with the help of dedicated software tools.
Generally, the NBR sets are defined taking into account the topology of the network under planning, possibly in combination with electromagnetic field propagation aspects.
Methods for defining the NBR sets in the network planning phase are for example described in the published European patent application EP 1 427 234; essentially, the aim of those methods is to define the NBR set for a generic network cell putting in it the cells that, most likely, will be destination cells of hand-over procedures from the considered cell, once the network will be deployed.